1. Field of the Invention
The present invention relates to a current-mode controlled DC-DC converter with a step-up chopper circuit employing peak-current-mode control, and particularly, to a current-mode controlled DC-DC converter for an electric vehicle.
2. Description of the Related Art
FIG. 1 is a circuit diagram illustrating a DC-DC converter according to a related art. This DC-DC converter is a step-up DC-DC converter including a DC power source Vdc1, transformers T3 and T4, a reactor L3, switches Q1 and Q2, diodes D3 and D4, a smoothing capacitor C1, and a control circuit 100.
The transformer T3 has a primary winding 5a (whose number of turns being np), a winding 5b (whose number of turns being np1) connected in series with the primary winding 5a, and a secondary winding 5c (whose number of turns being ns) electromagnetically coupled with the windings 5a and 5b. The transformer T4 is constituted like the transformer T3 and includes a primary winding 6a (whose number of turns being np), a winding 6b (whose number of turns being np1) connected in series with the primary winding 6a, and a secondary winding 6c (whose number of turns being ns) electromagnetically coupled with the windings 6a and 6b. 
Ends of the DC power source Vdc1 are connected to the drain and source of the switch Q1 through the primary winding 5a of the transformer T3. The switch Q1 may be a MOSFET. The ends of the DC power source Vdc1 are also connected to the drain and source of the switch Q2 through the primary winding 6a of the transformer T4. The switch Q2 may be a MOSFET. The winding 5b of the transformer T3, the diode D3, and the smoothing capacitor C1 form a first series circuit. One end of the first series circuit is connected to a connection point between the primary winding 5a of the transformer T3 and the drain of the switch Q1, and the other end of the first series circuit is connected to the source of the switch Q1. The winding 6b of the transformer T4, the diode D4, and the smoothing capacitor C1 form a second series circuit. One end of the second series circuit is connected to a connection point between the primary winding 6a of the transformer T4 and the drain of the switch Q2, and the other end of the second series circuit is connected to the source of the switch Q2.
Ends of a series circuit that includes the secondary winding 5c of the transformer T3 and the secondary winding 6c of the transformer T4 are connected to the reactor L3. According to an output voltage Vo from the smoothing capacitor C1, the control circuit 100 turns on/off the switches Q1 and Q2 with a phase difference of 180 degrees.
Operation of the DC-DC converter according to the related art illustrated in FIG. 1 will be explained. The control circuit 100 outputs a control signal Q1g to turn on the switch Q1 and pass a current through a path extending along a positive electrode of Vdc1, 5a, Q1, and a negative electrode of Vdc1. As a result, a current Q1i of the switch Q1 linearly increases. At this time, the secondary winding 5c of the transformer T3 generates a voltage to pass a current L3i through a path extending along 5c, L3, 6c, and 5c. 
The current L3i passes according to the low of equal ampere-turns, to accumulate energy in the reactor L3 and pass the same current through the secondary winding 6c of the transformer T4. As a result, the primary winding 6a and winding 6b of the transformer T4 induce voltages proportional to the numbers of turns thereof.
In a case where the transformer T4 has a turn ratio A as expressed by a relationship of A=(np+np1)/np, a current of Q1i/A (Q1i being the current of switch Q1) passes through the diode D4 through a path extending along the positive electrode of Vdc1, 6a, 6b, D4, C1, and the negative electrode of Vdc1. The current D4i of the diode D4 passes until when the switch Q2 turns on. The output voltage Vo of the smoothing capacitor C1 is the sum of a voltage (input voltage) of the DC power source Vdc1, a voltage generated by the primary winding 6a of the transformer T4, and a voltage generated by the winding 6b of the transformer T4.
A voltage generated by the transformer T4 is expressed by a relationship of A·Vdc1·D, where D is an ON duty of the switch Q1(D=Ton/T), Ton is an ON time of the switch Q1, and T is a switching period of the switch Q1. The output voltage Vo of the smoothing capacitor C1 is expressed as follows:Vo=Vdc1(1+A·D)Namely, the output voltage Vo is controllable by changing the ON duty D.
Thereafter, the control circuit 100 turns off the switch Q1 with the control signal Q1g, to pass a current D3i through a path extending along the positive electrode of Vdc1, 5a, 5b, D3, C1, and the negative electrode of Vdc1.
Then, the control circuit 100 provides a control signal Q2g to turn on the switch Q2 and pass a current through a path extending along the positive electrode of Vdc1, 6a, Q2, and the negative electrode of Vdc1. As a result, a current Q2i of the switch Q2 linearly increases. At this time, the secondary winding 6c of the transformer T4 generates a voltage to increasingly pass a current L3i through a path extending along 6c, 5c, L3, and 6c. 
The current L3i passes according to the low of equal ampere-turns, to accumulate energy in the reactor L3 and pass the same current through the secondary winding 5c of the transformer T3. As a result, the primary winding 5a and winding 5b of the transformer T3 induce voltages proportional to the numbers of turns thereof.
In a case where the transformer T3 has a turn ratio A as expressed by a relationship of A=(np+np1)/np, a current of Q2i/A (Q2i being the current of switch Q2) passes through the diode D3 and through a path extending along the positive electrode of Vdc1, 5a, 5b, D3, C1, and the negative electrode of Vdc1. The current D3i of the diode D3 passes until when the switch Q1 turns on. The output voltage Vo of the smoothing capacitor C1 is the sum of a voltage (input voltage) of the DC power source Vdc1, a voltage generated by the primary winding 5a of the transformer T3, and a voltage generated by the winding 5b of the transformer T3.
In this way, the multiphase, transformer-linked, step-up chopper circuit illustrated in FIG. 1 couples two independent phases with each other through the transformers. This configuration can perform a step-up operation with a single core instead of dual cores.
Another related art is Mark Hartman, “Design of current-mode controlled DC-DC converter,” Transistor Gijutsu, April 2004, pp. 213-222, CQ Publishing Co., Ltd. This related art achieves peak-current-mode control that turns on a switch, detects a current passing through a step-up reactor, and turns off the switch if a voltage corresponding to the detected current is equal to a control voltage. Namely, this related art restricts a peak value of a current signal detected from a step-up chopper circuit, thereby controlling the current passing therethrough.